The use of electronic article security systems for detecting and preventing theft or Unauthorized removal of articles or goods from retail establishments and/or other facilities, such as libraries, has become widespread. In general, such security systems employ a security tag or tag which is associated with or is secured to an article (or its packaging) of a type which is readily accessible to potential customers or facility users. Security tags may take on many different sizes, shapes and forms depending upon the particular type of security system in use, the type and size of the article, its packaging, etc. In general, such electronic article security systems are employed for detecting the presence (or absence) of a security tag and thus, a protected article, as the protected article passes through or near a surveilled security area or zone. In most cases, the surveilled security area is located at or near an exit or entrance to the retail establishment or other facility.
One such electronic article security system which has gained widespread popularity utilizes a security tag which includes a self-contained, operatively tuned or resonant circuit which resonates at a predetermined detection frequency. When an article having an attached security tag moves into or otherwise passes through the surveilled area, the tag is exposed to an electromagnetic field created by the security system. Upon being exposed to the electromagnetic field, a current is induced in the tag creating a field which changes the field created within the surveilled area. The magnitude and phase of the current induced in the tag is a function of the proximity of the tag to the security system, the frequency of the applied field, the resonant frequency of the tag, and the Q factor of the tag. The resulting change in the field created within the surveilled area because of the resonating security tag can be detected by the security system. Thereafter, the security system applies certain predetermined selection criteria to the detected signal to determine whether the change in the field within the surveilled area resulted from the presence of a tag or resulted from some other source. If the security system determines that the change in the field is the result of the presence of a security tag, it activates an alarm to alert appropriate security or other personnel.
While electronic article security systems of the type described above function very effectively, a limitation of the performance of such systems relates to false alarms. False alarms occur when the field created within the surveilled area is disturbed or changed by a source other than a security tag and the security system, after applying the predetermined selection criteria, still concludes that a security tag is present within the surveilled area and activates an alarm when in fact no security tag is actually present over the years, such systems have become quite sophisticated in the application of multiple selection criteria for security tag identification and in the application of statistical tests in the selection criteria applied to a suspected security tag signal. However, the number of false alarms is still unacceptably high in some applications. Accordingly, there is a need for a security tag for use in such electronic article security systems which provides more information than is provided by present security tags in order to assist such electronic article security systems in distinguishing signals resulting from the presence of a security tag within a surveilled area and similar or related signals which result from Other sources.
One method of providing additional information to the security system is to have two or more security tags each with a different resonant frequency secured to the article being protected. For example, the resonant frequency of a second tag could be offset from the resonant frequency of a first tag by a known ratio. In this manner, the simultaneous detection of two or more signals at specific predetermined separated frequencies each having the characteristics of a security tag signal would have a high probability of indicating the presence of the multiple security tags in the surveilled area since the probability of some other source or sources simultaneously generating each of the multiple signals at each of the predetermined frequencies is very small. It is generally known that when such security tags are placed in close proximity, they also share the magnetic flux generated by one another when current is induced in the tags. The sharing of the flux between the tags creates a coupling of the tags causing the tags to act as a load on one another. The additional loading prevents the tags from resonating at their design resonant frequencies. The tags must, therefore, be widely separated from each other.
The concept of utilizing multiple security tags at different frequencies on each article has not been generally accepted because of the requirement for physically separating the tags by a substantial distance in order to preclude the tags from interacting in such a way that the respective resonant frequencies and Q factors of the tags are detrimentally affected. Placing the security tags at a substantial distance from each other is disadvantageous because at best it requires separate tagging operations thereby substantially increasing the cost of applying the security tags. In addition, some articles are just not large enough to permit the two or more tags to be separated enough to preclude interaction. Separating the tags by a significant distance also affects the orientation and, therefore, the signal strength from the tags thereby limiting detectability of one or more of the tags.
The present invention comprises a multiple frequency security tag for use within an electronic article security system comprised of essentially two or more tags which are in close proximity to each other but in a specific predetermined spatial relationship in which there is zero or near zero coupling between the tags. The specific spatial relationship is one in which the tags partially overlap or overlie each other to the extent that the net flux generated from the coil of one of the tags is substantially zero within the area of the coil of the other tags and vice versa. In effect, with the tags partially overlying each other, flux generated from the current flowing through the coil of any one tag passes through the coils of the other tags in opposite directions so that the flux generated by the one tag passing through the coils of the other tags in a first direction is generally equal in magnitude but opposite in direction to the flux generated by the one tag passing through the coils of the other tags in the opposite direction. In this manner, the net flux flowing through the coils of the other tags from the one tag is zero or near zero and there is no substantial interaction between the tags to diminish the performance of any of the tags.